Biaxially oriented laminated polyester film for magnetic recording medium

ABSTRACT

The film of the present invention is a biaxially oriented laminated polyester film for magnetic recording medium consisting of a polyester layer A containing fine particles and laminated to at least one surface of a polyester layer B, wherein the fine particles in the polyester layer A are composed of 0.001 to 0.03% by weight of crosslinked polymer particles (a) having an average particle diameter (da) of 0.9 to 1.6 μm, 0.1 to 0.8% by weight of inert particles (b) having an average particle diameter (db) of 0.4 to 0.8 μm and 0.05 to 1.0% by weight of inert inorganic particles (c) having an average particle diameter (dc) of 0.01 to 0.3 μm and a Mohs hardness of 7 or above, and the ratio of the thickness TA (μm) of the polyester layer A to the average particle diameter da (μm) of the crosslinked polymer particle (TA/da) is in the range of from 0.4 to 2.0. The film has excellent windability, abrasion resistance, transportation durability and electromagnetic conversion characteristics and is producible at a low production cost and useful especially as a base film of a magnetic recording medium for high-speed duplicator.

DETAILED DESCRIPTION OF THE INVENTION

1. Technical Field

The present invention relates to a biaxially oriented laminatedpolyester film for magnetic recording medium. More particularly, itrelates to a biaxially oriented laminated polyester film for magneticrecording medium, having excellent windability, abrasion resistance anddurability in transportation as well as electromagnetic conversioncharacteristics, producible at a low production cost, and usefulespecially as a base film of a magnetic recording medium for ahigh-speed duplicator.

2. Background Arts

Biaxially oriented polyester films represented by polyethyleneterephthalate film are being widely used as a base film of a magneticrecording medium such as a magnetic tape owing to their excellentphysical and chemical properties.

The growth of the production of tapes recorded with information such ascinema picture is remarkable recently among various uses of magnetictapes. The production of recorded tapes was carried out formerly byduplicating from a master VTR (video tape recorder) to several thousandsof VTRs at a high speed, however, a new duplication technology has beendeveloped recently which comprises the superposition of a blank magnetictape to a mirror master tape recorded with the above information and theapplication of a magnetic field and heat to transfer the information tothe blank tape. The duplication speed of the method is extremely high.Accordingly, the required characteristics for the magnetic tape arevaried, and various new required characteristics have been developed forthe base film of the tape for the high-speed duplication use. Forexample, the tape is required to have improved windability in the formof a magnetic tape in the high-speed winding, close contact to themaster tape, smoothness of the film surface to improve theelectromagnetic conversion characteristics and abrasion resistance witha guide tape contacting with the tape in duplication.

The conventional base film of magnetic recording medium is incorporatedwith inert particles having large diameter to improve the windability byimproving the air-squeezing performance. The addition of a large amountof such inert particles having large diameter to a base film formsprotrusions on the surface of the magnetic layer by the particles, andthe transfer of the profile of the protrusions formed by thelarge-diameter inert particle to the magnetic layer surface in the caseof winding the tape in overlapped state to remarkably deteriorate thesurface properties of the magnetic layer of the magnetic tape. There areother problems such as the abrasion of the coarse inert particles bycontacting with the guide roll to generate white powder and causedrop-out defects.

The environment against the abrasion resistance and transport durabilityof magnetic tapes tends to be severe owing to e.g. the use of roughlysurface-finished cassette half and guide pin or the use of a plasticguide pin to reduce the cost of a magnetic recording and playingapparatus such as VTR, add the improvement for meeting the requirementsis still more keenly desired.

DISCLOSURE OF THE INVENTION

As a result of vigorous investigation for achieving the newcharacteristics required as a base film of a magnetic tape for theaforementioned high-speed duplicator, i.e. the improvement on thewindability at a high speed, the smoothness of the film surface and theabrasion resistance, etc., and for solving the problems to improve theabrasion resistance and the transportation durability so as to be usableeven on an inferior cassette half, guide pin, etc., the inventors of thepresent invention have found that the above problems can be solved byusing a laminated polyester film, adding three specific kinds ofparticles to the outermost layer constituting at least one of thesurfaces and selecting the ratio of the thickness of the outermost layerto the diameter of the particle having the largest average particlediameter to be fallen within a specific range and that a biaxiallyoriented laminated polyester film having extremely excellent totalperformance and producible at a low cost can be produced by thisprocess.

The object of the present invention is to provide a biaxially orientedlaminated polyester film for a magnetic recording medium havingexcellent windability, abrasion resistance, transportation durabilityand electromagnetic conversion characteristics, producible at a low costand useful as a base film of a magnetic recording medium for high-speedduplicators.

The other objects and advantages of the present invention will becomeclear from the following explanations.

The object of the present invention is achieved by a biaxially orientedlaminated polyester film for magnetic recording medium comprising abiaxially oriented polyester film having a polyester layer A containingfine particles laminated to at least one surface of a polyester layer Band characterized in that the fine particles in the polyester layer Aare composed of 0.001 to 0.03% by weight of crosslinked polymerparticles (a) having an average particle diameter (da) of 0.9 to 1.6 μm,0.1 to 0.8% by weight of inert particles (b) having an average particlediameter (db) of 0.4 to 0.8 μm and 0.05 to 1.0% by weight of inertinorganic particles (c) having an average particle diameter (dc) of 0.01to 0.3 μm and a Mohs hardness of 7 or above and that the ratio of thethickness TA (μm) of the polyester layer A to the average particlediameter da (μm) of the crosslinked polymer particles (a) TA/da is from0.4 to 2.0.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a schematic diagram of an apparatus for the measurement ofkinetic friction coefficient. In the FIG. 1, the sign 1 is a feed reel,2 is a tension controller, 3,5,6,8,9 and 11 are free rollers, 4 is atension detector (inlet side), 7 is a stationary rod, 10 is a tensiondetector (outlet side), 12 is a guide roller and 13 is a take-up reel.

BEST MODE FOR CARRYING OUT THE INVENTION

The biaxially oriented laminated polyester film of the present inventionis necessary to have the above-mentioned characteristics.

In the present invention, the polyester is a saturated aromaticpolyester produced by using an aromatic dicarboxylic acid as a maindicarboxylic acid component and an aliphatic glycol as a main glycolcomponent. The polyester is an essentially linear polymer and capable offorming a film especially by melt-forming.

The aromatic dicarboxylic acid is e.g. terephthalic acid,naphtahlenedicarboxylic aid, isophthalic acid, diphenoxyethanedicarboxylic acid, diphenyldicarboxylic acid, diphenyl etherdicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenyl ketonedicarboxylic acid and anthracenedicarboxylic acid and the aliphaticglycol is e.g. a polymethylene glycol having a carbon number of from 2to 10 such as ethylene glycol, trimethylene glycol, tetramethyleneglycol, pentamethylene glycol, hexamethylene glycol and decamethyleneglycol, an aliphatic diol such as cyclohexane-dimethanol, etc.

In the present invention, the polyester is preferably a polymer havingan alkylene terephthalate and/or an alkylene-2,6-naphthalate as a mainconstituent component.

Especially preferable polyesters among the above polyesters arepolyethylene terephthalate, polyethylene-2,6-naphthalate and a copolymercontaining terephthalic acid or 2,6-naphthalene-dicarboxylic acidaccounting for not less than 80 mol % of the total dicarboxylic acidcomponent and ethylene glycol accounting for not less than 80 mol % ofthe total glycol component. In this case, less than 20 mol % of thetotal acid component may be the above aromatic dicarboxylic acid otherthan terephthalic acid or 2,6-naphthalenedicarboxylic acid, an aliphaticdicarboxylic acid such as adipic acid or sebacic acid or an alicyclicdicarboxylic acid such as cyclohexane-1,4-dicarboxylic acid, etc. Lessthan 20 mol % of the total glycol component may be the above glycolsother than ethylene glycol or an aromatic diol such as hydroquinone,resorcin or 2,2-bis(4-hydroxyphenyl)propane, an aliphatic diol havingaromatic ring such as 1,4-dihydroxydimethylenebenzene, a polyalkyleneglycol (polyoxyalkylene glycol) such as polyethylene glycol,polypropylene glycol or polytetramethylene glycol, etc.

Included in the polyesters of the present invention are polymerscopolymerized or bonded with not more than 20 mol % of a componentderived from an oxycarboxylic acid, for example, an aromatic oxy acidsuch as hydroxybenzoic acid or an aliphatic oxy acid such asω-hydroxycaproic acid based on the total amount of the dicarboxylic acidcomponent and the oxycarboxylic acid component.

The polyester of the present invention includes a copolymercopolymerized with three or more functional polycarboxylic acid orpolyhydroxy compound such as trimellitic acid or pentaerythritol in anamount to get an essentially linear polymer, for example, 2 mol % orless based on the total acid component.

The above polyesters are known in itself and producible by knownprocesses.

The biaxially oriented laminated polyester film of the present inventionis composed of a polyester layer A laminated to at least one surface ofa polyester layer B. The laminate preferably has a double-layerstructure consisting of layer A/layer B or a triple-layer structureconsisting of layer A/layer B/layer A, however, the triple-layerstructure is more preferable because the recovered scrap film generatedin the polyester film manufacturing process can be reused as or blendedto the polyester film layer B.

The above-mentioned polyester can be used as the polyester for the layerA and the layer B. The polyesters of the layer A and the layer B may bethe same or different and the use of the same polyesters is preferable.

The polyester layer A of the biaxially oriented laminated polyester filmof the present invention contains three kinds of inert particles (a),(b) and (c) having different average particle diameters at the sametime.

The inert particle (a) is a crosslinked polymer particle having anaverage particle diameter (da) falling within the range of 0.9 to 1.6 μmand its content is in the range of 0.001 to 0.03% by weight. When theaverage particle diameter (da) and the content of the particle (a) aresmaller than the above ranges, the improving effect on the windabilityof the film becomes insufficient. On the contrary, when the averageparticle diameter (da) and the content are larger than the above ranges,the electromagnetic conversion characteristics of the produced magnetictape and the abrasion resistance of the film are deteriorated toundesirable levels. The average particle diameter (da) of the particle(a) is preferably in the range of 1.0 to 1.5 μm, more preferably in therange of 1.1 to 1.4 μm. The content of the particle (a) is preferably inthe range of 0.003 to 0.025% by weight, more preferably in the range of0.005 to 0.02% by weight.

The crosslinked polymer particle (a) is preferably at least one kind ofparticle selected from crosslinked silicone resin particle andcrosslinked polystyrene resin particle because these resins have highaffinity to polyester and the particle is soft to easily absorb theshock applied to protrusions and to resist the falling off of theprotrusion by the collision at a high speed.

The crosslinked polymer particle (a) preferably has an apparent Young'smodulus of the particle of 10 to 100 kg/mm², more preferably 10 to 50kg/mm². When the apparent Young's modulus is smaller than 10 kg/mm², theparticles incorporated in the film are deformed by the stress in thedrawing process to fail in getting high protrusions necessary forimparting the film with windability. On the contrary, the particlebecomes too hard to keep the desirable impact strength and is liable tobe fallen off when the apparent Young's modulus exceeds 100 kg/mm².

The inert particle (b) has an average particle diameter (db) of from 0.4to 0.8 μm and its content is in the range of 0.1 to 0.8% by weight. Whenthe average particle diameter (db) and the content of the particle aresmaller than the above ranges, the slipperiness of the film becomes poorto make the winding of the film difficult and cause unstabletransportation of the tape in a VTR. On the contrary, the abrasionresistance is deteriorated when these values are larger than the aboveranges. The average particle diameter (db) of the inert particle (b) ispreferably in the range of 0.4 to 0.7 μm, more preferably in the rangeof 0.4 to 0.6 μm. The content of the inert particle (b) is preferably inthe range of 0.15 to 0.7% by weight, more preferably in the range of 0.2to 0.6% by weight.

The inert particle (b) is an inorganic particle and the preferableexamples are (1) silicon dioxide (including hydrated silicon dioxide,silica sand, quartz, etc.), (2) alumina having various crystalmorphology, (3) a silicate having an SiO₂ content of 30% by weight ormore (e.g. amorphous or crystalline clay mineral, aluminosilicate(including calcined material and hydrated material), chrysotile, zirconor fly ash), (4) oxide of Mg, Zn, Zr or Ti, (5) sulfate of Ca or Ba, (6)phosphate of Li, Ba or Ca (including monohydrogen salt and dihydrogensalt), (7) benzoate of Li, Na or K, (8) terephthalate of Ca, Ba, Zn orMn, (9) titanate of Mg, Ca, Ba, Zn, Cd, Pb, Sr, Mn, Fe, Co or Ni, (10)chromate of Ba or Pb, (11) carbon (e.g. carbon black or graphite), (12)glass (e.g. glass powder or glass beads), (13) carbonate of Ca or Mg,(14) fluorite and (15) Zn. Calcium carbonate is most preferable amongthe above materials.

The inert particle (c) is an inert inorganic particle having a Mohshardness of 7 or above and an average particle diameter (dc) of 0.01 to0.3 μm. The content of the particle (c) is in the range of 0.05 to 1.0%by weight. An inert inorganic particle (c) having a Mohs hardness ofsmaller than 7 is undesirable because of its insufficient scratchresistance. The inert inorganic particle having a Mohs hardness of 7 orabove is preferably an agglomerated particle composed of aluminum oxide(alumina) or spinel-type oxide and having an average agglomerationdegree of 2 to 20. The scratch resistance of the film becomesundesirably poor when the average agglomeration degree is out of theabove range. The average agglomeration degree is preferably in the rangeof 2 to 15, more preferably 2 to 10 and most preferably 2 or above andless than 5 from the viewpoint of the improving effect on the scratchresistance. When the inert inorganic particle (c) is an agglomeratedparticle composed of aluminum oxide (alumina), the particle preferablyhas θ-type crystal structure to increase the effect for improving thescratch resistance. When the inert inorganic particle (c) is aspinel-type oxide, it is preferably MgAl₂O₄ to increase the scratchresistance improving effect.

When the average particle diameter (dc) and the content of the inertinorganic particle (c) are smaller than the above ranges, the improvingeffect on the scratch resistance becomes undesirably insufficient. Onthe contrary, the use of the particle having these values exceeding theabove ranges is also unfavorable because of insufficient scratchresistance improving effect and deteriorated abrasion resistance. Theaverage particle diameter (dc) of the inert inorganic particle (c) ispreferably within the range of from 0.03 to 0.25 μm, more preferablyfrom 0.05 to 0.2 g m. The content of the particle (c) is preferablywithin the range of from 0.1 to 0.7% by weight, more preferably from0.15 to 0.4% by weight and most preferably 0.2% by weight or above andless than 0.25% by weight.

The biaxially oriented laminated polyester film of the present inventionis necessary to have a TA/da ratio falling within the range of from 0.4to 2.0, wherein TA (μm) is the thickness of the polyester layer A and da(μm) is the average particle diameter of the crosslinked polymerparticle (a). When the TA/da ratio is smaller than the above range, thefalling-off of the particle is liable to occur to deteriorate theabrasion resistance. On the contrary, the film having the ratio largerthan the above range has deteriorated electromagnetic conversioncharacteristics owing to the roughening of the film surface by theinfluence of particles (a) existing in a layer deeper than twice theparticle diameter. The value of the TA/da ratio is preferably within therange of from 0.6 to 1.9, more preferably from 0.8 to 1.8.

The polyester layer B of the biaxially oriented laminated polyester filmof the present invention is not necessary to contain inert particles,however, it is preferable that the layer B contains inert particleshaving an average particle diameter of 0.4 μm or above, such as theaforementioned crosslinked polymer particle (a), inert particles (b),etc., in an amount (CB) satisfying the following formula to enable thereclamation and use of the scrap film generated in the polyester filmmanufacturing process for the production of the layer B.${CB} = {{CA} \times \frac{A}{B} \times \frac{R}{1 - R}}$

wherein CA is the content (% by weight) of inert particles having anaverage particle diameter of 0.4 μm or above in the polyester layer A,CB is the content (% by weight) of inert particles having an averageparticle diameter of 0.4 μm or above in the polyester layer B, dA is thetotal thickness (μm) of the polyester layer A, dB is the total thickness(μm) of the polyester layer B and R is a number of from 0.3 to 0.07.

When the value R of the above formula is larger than 0.7 or smaller than0.3, the variation of the content of inert particles having an averageparticle diameter of 0.4 μm or above in the polyester layer B becomeslarge and, consequently, the variation of the surface roughness of thepolyester layer A is also increased. The preferable value of R isbetween 0.4 and 0.6. Fine inert particles having an average particlediameter of 0.4 μm or under gives little influence on the surface of thepolyester layer A even if the particles are present in the polyesterlayer B.

The center-line average roughness RaA of the surface of the polyesterlayer A of the biaxially oriented laminated polyester film of thepresent invention is preferably 8 to 18 nm, more preferably 9 to 17 nmand especially 10 to 16 nm. When the center line average roughness RaAis smaller than 8 nm, the surface is too smooth to give sufficientimproving effect on the windability and the transportation durabilityand when it is larger than 18 nm, the electromagnetic conversioncharacteristics of the magnetic tape manufactured from the film isdeteriorated owing to excessive roughness of the surface.

The biaxially oriented laminated polyester film of the present inventionhas a windability index of preferably 100 or below at a winding speed of200 m/min. The film having a windability index of 100 or below ispreferable because of remarkable improving effect on the windability inthe case of using the film as a base film of a tape for a high-speedduplicator. On the contrary, a film having a windability index of largerthan 100 is undesirable because the film gives a wound roll having anunsatisfactory shape such as rugged side face of the film roll and, inextreme cases, roll collapse may occur during the winding operation. Thewindability index at a winding speed of 200 m/min is more preferably 85or below, especially 70 or below.

The biaxially oriented laminated polyester film of the present inventionhas a total film thickness of preferably 5 to 25 μm, more preferably 10to 20 μM.

The Young's modulus of the film in longitudinal direction is preferably400 kg/mm² or above, more preferably 450 kg/mm² or above and preferablynot larger than 600 kg/mm². The Young's modulus in lateral direction ispreferably 500 kg/mm² or above, more preferably 600 kg/mm² or above andpreferably not larger than 800 kg/mm². A film having longitudinal andlateral Young's moduli falling within the above ranges is preferablebecause such film can sufficiently cope with the reduction of the filmthickness required for a long-playing tape.

The biaxially oriented polyester film of the present invention can bemanufactured principally by a known process or a process accumulated inthe industry. For example, it can be manufactured by producing alaminated undrawn film and biaxially orienting the film. The laminatedundrawn film can be produced by a laminated film production processaccumulated in the industry. For example, the laminated undrawn film canbe produced by laminating a surface-forming film layer (polyester layerA) to a core-forming film layer (polyester layer B) in molten state orin a cooled and solidified state. Concretely, it can be produced bycoextrusion, extrusion lamination, etc.

The laminated undrawn film produced by the above process can beconverted to a biaxially oriented film by a process accumulated in theindustry for the production of a biaxially oriented film. For example, alaminated undrawn film is drawn in one direction (longitudinal directionor lateral direction) at a draw ratio of 3.0 to 4.0 and a temperaturebetween Tg −10° C. and Tg +70° C. (Tg is the glass transitiontemperature of the polyester) and further drawing in a directionperpendicular to the first drawing direction (when the film is drawn inlongitudinal direction at the first stage, the second stage drawing isin lateral direction) at a draw ratio of 3.6 to 4.5 and a temperaturebetween Tg° C. and Tg+70° C. The draw ratio in each direction ispreferably selected to give a Young's modulus satisfying theaforementioned requirement and the areal draw ratio is preferablyselected within the range of from 13 to 18. The drawing can be carriedout by simultaneous biaxial drawing or a consecutive biaxial drawing.The biaxially oriented film may be heat-treated at a temperature between(Tg+70)° C. and Tm° C. For example, a laminated polyethyleneterephthalate film is preferably heat-set at 190 to 230° C. Theheat-setting time is e.g. 1 to 60 seconds.

Since the biaxially oriented polyester film of the present inventioncontains three specific kinds of particles in the outermost layerconstituting at least one of the surfaces and the ratio of the thicknessof the outermost layer to the particle diameter of the particle havinglargest particle diameter is selected to be fallen in a specific range,it is a film extremely useful for a magnetic recording use, havingexcellent performance comprising improved high-speed windabilityrequired as a base film of a magnetic recording medium for high-speedduplicator, smoothness of the base surface and abrasion resistance aswell as improved abrasion resistance and transportation stability on aroughly surface-finished cassette half and guide pin, etc., producibleat a low cost and having excellent total performance.

Various physical properties and characteristics referred in thespecification of the present invention were measured and defined by thefollowing methods.

(1) Average particle diameter (dp) of particle

The average particle diameter was measured by using a CentrifugalParticle Size Analyzer Type CP-50 manufactured by Shimadzu Corp. Acumulative curve between the particle diameter and the amount of theparticle was drawn based on the obtained centrifugal precipitationcurve, and the particle diameter corresponding to 50 mass % was readfrom the cumulative curve and used as the average particle diameter(“Particle Size Measuring Technique”, a monograph published by NikkanKogyo Shimbun Ltd., 1975, pp.242-247).

(2) Apparent Young's modulus of particle

A diamond indenter was lowered at a constant loading rate (29 mgf/sec)by an ultramicro-compression tester MCTM-201 manufactured by ShimadzuCorp. to apply an external force to a single particle. The apparentYoung's modulus Y was calculated from the load (kgf) at the breakage ofthe particle, the displacement Z (mm) of the indenter at the breakage ofthe particle and the particle diameter d (mm) according to the followingformula. Similar operations were repeated 10 times and the average ofthe ten data was adopted as the apparent Young's modulus of theparticle.

Y=2.8P/πdZ

(3) Average agglomeration degree of particle

A particle-containing film was sliced in the direction of cross-sectionto obtain an ultra-thin slice of 100 nm thick. The particles in theslice were observed with a transmission electron microscope (e.g.JEM-1200EX manufactured by JEOL Ltd.) to find undividable smallestparticles (primary particles). The number of primary particlesconstituting each of the agglomerated particles (secondary particles)were counted on 100 particles from the observation photograph and thetotal number of the primary particles was divided by the number of theobserved secondary particles to get the average agglomeration degree.

(4) Surface roughness (Ra) of film

Center-line average roughness (Ra) defined by JIS B0601 was measured bya tracer-type surface roughness tester of Kosaka Kenkyusho Ltd.(SURFCORDER SE-30C) under the following conditions.

(a) Tip radius of the tracer: 2 μm

(b) Measuring pressure: 30 mg

(c) Cut-off: 0.25 mm

(d) Measuring length: 2.5 mm

(e) Processing of the data: Measurements were repeated 6 times on thesame specimen, the largest one was omitted from the data and the averageof the remaining 5 data was used as the surface roughness.

(5) Calender abrasivity

The abrasivity of the transporting face of a base film was estimated byusing a three-stage miniature super-calender. The calender hadthree-stages comprising nylon rolls and steel rolls and the film wastransported at a film speed of 100 m/min and calendered at a calenderingtemperature of 80° C. under a linear pressure of 200 kg/cm. Theabrasivity of the base film was estimated by the stain attached to thetop roller of the calender after transporting 4,000 m of the film intotal.

<5-stage judgement>

Class 1: Absolutely no contamination of nylon roll

Class 2: Little contamination of nylon roll

Class 3: Some contamination on nylon roll easily erasable by rubbingwith dry cloth

Class 4: Contamination of nylon roll scarcely erasable with dry clothbut erasable with a solvent such as acetone

Class 5: Considerable contamination of nylon roll scarcely removablewith solvents

(6) Blade abrasion resistance

An edge of a blade (blade for an industrial blade tester manufactured byGKI, U.S.A.) was perpendicularly applied to a film cut to 1/2 inch wideand pressed into the film to the depth of 2 mm and the film wastransported (under friction) at a speed of 100 m/min under an inlettension Ti of 50 g in an environment of 20° C. and 60% relativehumidity. The abrasion resistance was estimated by the amount of abradedpowder attached to the blade after the transportation of 100 m of thefilm.

<Judgement>

⊚: The width of abraded powder attached to the edge of the blade issmaller than 0.5 mm

∘: The width of abraded powder attached to the edge of the blade is 0.5mm or above and smaller than 1.0 mm.

Δ: The width of abraded powder attached to the edge of the blade is 1.0mm or above and smaller than 2.0 mm.

X: The width of abraded powder attached to the edge of the blade is 2.0mm or above.

(7) Scratch resistance and abrasion resistance under high-speedtransportation

These properties were measured by the following method using theapparatus shown in the FIG. 1.

In the FIG. 1, the sign 1 is a feed reel, 2 is a tension controller,3,5,6,8,9 and 11 are free rollers, 4 is a tension detector (inlet side),7 is a stationary rod, 10 is a tension detector (outlet side), 12 is aguide roller and 13 is a take-up reel.

A film slit to a width of ½ inch was brought into contact with thestationary rod 7 at a contacting angle θ of 60° , and 200 m of the filmwas transported in an environment of 20° C. and 60% humidity at a speedof 300 m/min while keeping the inlet-side tension to 50 g.

The abraded powder attached to the stationary rod 7 and the scratch onthe film formed after the transportation were observed to estimate thescratch resistance and the abrasion resistance.

There are three methods according to the kinds of the stationary rod.The method A uses a thoroughly polished 6φ tape guide made of SUS304(surface roughness Ra=0.015 μm), the method B uses a 6φ tape guideproduced by bending a sintered SUS plate in cylindrical form and havinginsufficiently finished surface (surface roughness Ra=0.15 μm), and themethod C uses a 6φ tape guide made of a polyacetal containing carbonblack.

<Judgement of abraded powder>

⊚: Abraded powder is absolutely unobservable.

∘: Abraded powder is faintly observable.

Δ: The presence of abraded powder is clearly observable.

X: Considerable amount of powder is attached to the rod.

<Judgement of scratch resistance>

⊚: Scratch is absolutely unobservable.

∘: One to five scratches are observable.

Δ: Six to fifteen scratches are observable.

X: The number of observable scratches is 16 or more.

(8) Kinetic friction coefficient (μk) and scratch resistance at repeatedlow-speed transportation

These properties were measured by the following method using theapparatus shown in the FIG. 1.

The non-magnetic face of a magnetic tape was brought into contact withthe stationary rod 7 at an angle 0=(152/180)π radian (152° ) andtransported (under friction) at a speed of 200 cm/min in an environmentof 20° C. and 60% relative humidity. The tension controller 2 wascontrolled to adjust the inlet-side tension T1 to 50 g and theoutlet-side tension (T2: g) was detected by the outlet-side tensiondetector after reciprocating the film 50 times. The kinetic frictioncoefficient μk was calculated by the following formula.

μk=(2.303/θ)log(T2/T1)=0.868log(T2/35)

When the kinetic friction coefficient (μk) is 0.25 or above, thetransportation of the tape becomes unstable when the tape is repeatedlytransported in a VTR, and the film having a kinetic friction coefficientexceeding the above value is judged to have poor transportationdurability.

There are three methods according to the kinds of the stationary rod.The method A uses a thoroughly polished 6φ tape guide made of SUS304(surface roughness Ra=0.015 μm), the method B uses a 6φ tape guideproduced by bending a sintered SUS plate in cylindrical form and havinginsufficiently finished surface (surface roughness Ra=0.15 μm), and themethod C uses a 6φ tape guide made of a polyacetal containing carbonblack.

The scratch resistance of the tape was judged by the following criterionon the scratch of the non-magnetic face of the tape after thetransportation test.

<Judgement of scratch resistance>

⊚: Scratch is absolutely unobservable.

⊚: One to five scratches are observable.

Δ: Six to fifteen scratches are observable.

X: The number of observable scratches is 16 or more.

The magnetic tape was manufactured by the following method.

One hundred (100) parts by weight (hereinafter abbreviated as parts) ofγ-Fe₂O₃ were kneaded with the following composition in a ball-mill for12 hours.

Polyester urethane 12 parts Vinyl chloride-vinyl acetate-maleicanhydride copolymer 10 parts α-Alumina 5 parts Carbon black 1 part Butylacetate 70 parts Methyl ethyl ketone 35 parts Cyclohexanone 100 parts.

After the kneading and dispersing operation, the mixture was furtherincorporated with

A fatty acid: oleic acid 1 part A fatty acid: palmitic acid 1 part Afatty acid ester (amyl stearate) 1 part

and kneaded for 10 to 30 minutes. The mixture was added with 7 parts of25% ethyl acetate solution of a triisocyanate compound and subjected tohigh-speed shearing dispersion treatment for 1 hour to obtain a magneticcoating liquid.

The obtained coating liquid was applied to a polyester film in an amountto get a dried film thickness of 3.5 μm.

The coated film was oriented in a DC magnetic field and dried at 100° C.The dried film was calendered and slit to ½ inch width to obtain amagnetic tape.

(9) Windability index

A film of ½ inch wide was passed through the apparatus shown in the FIG.1 by-passing the stationary rod 7 and 200 m of the film was transportedat a speed of 200 m/min in an environment of 20° C. and 60% relativehumidity while detecting the edge position of the film with a CCD cameraat a position immediately before winding the film on the take-up reel13.

The fluctuation of the edge position was drawn in the form of wave withtime and the windability index was calculated from the wave by thefollowing formula.$\text{Windability Index} = {\sqrt{\frac{1}{l}}{\int_{0}^{l}{{f(x)}^{2}\quad {x}}}}$

wherein l is measurement time (second) and x is the fluctuation (μ_m) ofthe edge.

(10) Windability

The magnetic tape manufactured by the above method was passed throughthe apparatus shown in the FIG. 1 by-passing the stationary rod 7 and500 m of the tape was transported at a speed of 400 m/min. Thewindability was evaluated by the winding performance at the side of thetake-up reel and the roll shape of the wound magnetic tape roll.

<Judgement>

∘: The edge displacement of the wound roll is 1 mm or less.

Δ: The edge displacement of the wound roll exceeds 1 mm.

X: The tape is unwindable.

(11) Electromagnetic conversion characteristics

A VTR of VHS-system (BR6400, Victor Company of Japan, Ltd.) wasmodified, a 4MHz sinusoidal wave was inputted to a recording andreproducing head through an amplifier, recorded on a magnetic tape andreproduced and the reproduced signal was inputted to a spectrumanalyzer. The noise generated at the frequency separated from the 4 MHzcarrier signal by 0.1 MHz was measured and the carrier-to-noise ratio(C/N) was expressed by dB unit. The magnetic tape produced above wasmeasured by this process, the C/N ratio of the tape manufactured by theComparative Example 17 was used as the standard (±0 dB) and thedifference from the standard magnetic tape was used as theelectromagnetic conversion characteristics.

EXAMPLES

The present invention is further described in detail by the followingExamples.

Examples 1 to 13 and Comparative Examples 1 to 16

Polyethylene terephthalates for the polyester layer A having anintrinsic viscosity of 0.56 (o-chlorophenol, 35° C.) were produced bypolymerizing dimethyl terephthalate and ethylene glycol by conventionalmethod using manganese acetate as a transesterification catalyst,antimony trioxide as a polymerization catalyst, phosphorous acid as astabilizer and particulate additives for the polyester layer A shown inthe Tables 1 and 3 as lubricants.

A polyethylene terephthalate produced by the process same as the abovewithout adding the particles was used as the polymer for the polyesterlayer B.

Pellets of these polyethylene terephthalate resins were dried at 170° C.for 3 hours, supplied to the hoppers of two extruders, melted at meltingtemperature of 280 to 300° C., extruded through a multi-manifoldcoextrusion die on a rotary cooling drum having a surface finish ofabout 0.3s and a surface temperature of 20° C. in a state laminating thelayers A to both surfaces of the layer B to obtain an undrawn laminatedfilm having a thickness of 200 μm.

The undrawn laminated film produced by the above process was preheatedat 75° C., drawn 3.2 times between a low-speed roll and a high-speedroll under heating with three IR heaters having a surface temperature of800° C. and placed 15 mm above the film, supplied to a stenter and drawn4.3 times in lateral direction at 120° C. The obtained biaxiallyoriented film was heat-set at 205° C. for 5 seconds to obtain a heat-setbiaxially oriented laminated polyester film having a thickness of 14 μm.The film had a longitudinal Young's modulus of 460 kg/mm² and a lateralYoung's modulus of 700 kg/mm².

The thickness of each layer was adjusted by adjusting the extrusionrates of two extruders and the width of the flow channel and wasdetermined by the fluorescent X-ray analysis in combination with theobservation of the boundary on the cross-section of a thinly sliced filmwith a transmission electron microscope.

Comparative Example 17

A single-layer biaxially oriented polyester film was produced by using apolyethylene terephthalate containing the particles described in theTable 3 by a method similar to the above Examples except for the use ofan ordinary single-layer die for extrusion.

The characteristics of the film produced by the process are shown in theTables 2 and 4. It is apparent from the Tables 2 and 4 that the film ofthe present invention has extremely excellent overall characteristicscomprising the excellent electromagnetic conversion characteristics,windability and abrasion resistance and the excellent scratchresistance, abrasion resistance and transportation durability to variouskinds of tape guides.

TABLE 1 Inert particle in layer A Crosslinked polymer particle A Inertparticle B Inert inorganic particle C Kind of particle, Apparent Kind ofparticle, Kind of particle, Average agglom- Average particle Content ofYoung's modulus Average particle Content of Average particle Content oferation degree diameter (da) particle of particle diameter (db) particlediameter (dc) particle of particle (μm) (%) (kg/mm²) (μm) (%) (μm) (%)(number) Example 1 Silicone resin 0.01 50 Calcium carbonate 0.2θ-Aluminum oxide 0.2 4.5 1.2 0.6 0.1 Example 2 Silicone resin 0.02 50Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.5 1.2 0.6 0.1 Example 3Silicone resin  0.005 50 Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.51.2 0.6 0.1 Example 4 Silicone resin 0.01 50 Calcium carbonate 0.2θ-Aluminum oxide 0.2 4.5 1.5 0.6 0.1 Example 5 Crosslinked 0.01 25Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.5 polystyrene 1.2 0.6 0.1Example 6 Silicone resin 0.01 50 Calcium carbonate 0.2 Spinel oxide 0.24.2 1.2 0.6 (MgAl₂O₄) 0.1 Example 7 Silicone resin 0.01 50 Calciumcarbonate 0.2 θ-Aluminum oxide 0.15 4.5 1.2 0.6 0.1 Spinel oxide 0.054.2 (MgAl₂O₄) 0.1 Example 8 Silicone resin 0.01 50 Calcium carbonate 0.2θ-Aluminum oxide 0.2 12.4 1.2 0.6 0.1 Example 9 Silicone resin 0.01 50Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.5 1.2 0.6 0.1 Example 10Silicone resin 0.01 50 Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.51.2 0.6 0.1 Example 11 Silicone resin 0.01 50 Calcium carbonate 0.2θ-Aluminum oxide 0.2 4.5 1.2 0.6 0.1 Example 12 Silicone resin 0.01 50Calcium carbonate 0.2 Spinel oxide 0.2 4.2 1.2 0.6 (MgAl₂O₄) 0.1 Example13 Silicone resin 0.01 50 Calcium carbonate 0.2 Spinel oxide 0.2 4.2 1.20.6 (MgAl₂O₄) 0.1

TABLE 2 Thick ness Thick Calen- Electro- of ness der Blade magneticLayer of abrasion abra- High speed transportation Low speed repeatedtransportation conversion A Layer resis- sion Scratch Abrasion ScratchWind- character- (T_(A)) B T_(A)/ Ra^(A) tance resis- resistanceresistance resistance Kinetic μk ability Wind- istics μm μm d_(a) nm(class) tance A B C A B C A B C A B C index ability C/N Ex- 2.0 10.0 1.714 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.20 60 ∘ +1.8 ample 1 Ex- 2.0 10.01.7 15 2 ∘ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.20 0.20 0.19 50 ∘ +1.5 ample 2 Ex- 2.010.0 1.7 14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.22 0.22 0.22 70 ∘ +1.9 ample 3 Ex-2.0 10.0 1.7 15 2 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.19 50 ∘ +1.6 ample 4Ex- 2.0 10.0 1.3 14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.21 60 ∘ +1.8 ample5 Ex- 2.0 10.0 1.7 14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.20 60 ∘ +1.8ample 6 Ex- 2.0 10.0 1.7 14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.20 60 ∘+1.8 ample 7 Ex- 2.0 10.0 1.7 14 1 ⊚ ⊚ ⊚ ∘ ⊚ ⊚ ∘ ⊚ ⊚ ∘ 0.20 0.20 0.20 60∘ +1.7 ample 8 Ex- 1.5 11.0 1.3 13 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.2060 ∘ +2.5 ample 9 Ex- 1.0 12.0 0.8 11 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.22 0.220.21 70 ∘ +3.1 ample 10 Ex- 0.6 12.8 0.5  9 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.240.23 0.23 90 ∘ +3.4 ample 11 Ex- 1.5 11.0 1.3 13 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚0.21 0.21 0.20 60 ∘ +2.4 ample 12 Ex- 1.0 12.0 0.8 11 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚⊚ ⊚ 0.22 0.22 0.21 70 ∘ +3.0 ample 13

TABLE 3 Inert particle in layer A Crosslinked polymer particle A Inertparticle B Inert inorganic particle C Kind of particle, Apparent Kind ofparticle, Kind of particle, Average agglom- Average particle Content ofYoung's modulus Average particle Content of Average particle Content oferation degree diameter (da) particle of particle diameter (db) particlediameter (dc) particle of particle (μm) (%) (kg/mm²) (μm) (%) (μm) (%)(number) Comparative — — — Calcium carbonate 0.2 θ-Aluminum oxide 0.24.5 Example 1 0.6 0.1 Comparative — — — Calcium carbonate 0.2 θ-Aluminumoxide 0.2 4.5 Example 2 0.4 0.1 Comparative Silicone resin 0.01 50 — —θ-Aluminum oxide 0.2 4.5 Example 3 1.2 0.1 Comparative Silicone resin0.01 50 Calcium carbonate 0.2 — — — Example 4 1.2 0.6 ComparativeSilicone resin 0.01 50 Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.5Example 5 0.7 0.6 0.1 Comparative Silicone resin 0.03 50 Calciumcarbonate 0.2 θ-Aluminum oxide 0.2 4.5 Example 6 2.0 0.6 0.1 ComparativeSilicone resin 0.0005 50 Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.5Example 7 1.2 0.6 0.1 Comparative Silicone resin 0.05 50 Calciumcarbonate 0.2 θ-Aluminum oxide 0.2 4.5 Example 8 1.2 0.6 0.1 ComparativeSpherical Silica 0.01 200  Calcium carbonate 0.2 θ-Aluminum oxide 0.24.5 Example 9 1.2 0.6 0.1 Comparative Silicone resin 0.01 50 Calciumcarbonate 0.2 Spherical silica 0.2 1.5 Example 10 1.2 0.6 0.1Comparative Silicone resin 0.01 50 Calcium carbonate 0.2 θ-Aluminumoxide 0.2 1.7 Example 11 1.2 0.6 0.1 Comparative Silicone resin 0.01 50Calcium carbonate 0.2 θ-Aluminum oxide 0.2 25.3  Example 12 1.2 0.6 0.1Comparative Silicone resin 0.01 50 Calcium carbonate 0.2 θ-Aluminumoxide 0.2 4.5 Example 13 1.0 0.8 0.1 Comparative Silicone resin 0.01 50Calcium carbonate 1.2 θ-Aluminum oxide 0.2 4.5 Example 14 1.2 0.6 0.1Comparative Silicone resin 0.01 50 Calcium carbonate 0.2 θ-Aluminumoxide 0.2 4.5 Example 15 1.2 0.6 0.1 Comparative Silicone resin 0.02 50Calcium carbonate 0.2 θ-Aluminum oxide 0.2 4.5 Example 16 1.2 0.6 0.1Comparative Silicone resin 0.02 50 Calcium carbonate 0.2 θ-Aluminumoxide 0.2 4.5 Example 17 1.2 0.6 0.1

TABLE 4 Thick ness Thick Calen- Electro- of ness der Blade magneticLayer of abrasion abra- High speed transportation Low speed repeatedtransportation conversion A Layer resis- sion Scratch abrasion ScratchWind- character- (T_(A)) B T_(A)/ Ra^(A) tance resis- resistanceresistance resistance Kinetic μk ability Wind- istics μm μm d_(a) nm(class) tance A B C A B C A B C A B C index ability C/N Com- 2.0 10.0 —14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.21 150 Δ +1.9 para Ex. 1 Com- 2.010.0 — 11 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.23 0.23 0.24 200 x +2.5 para Ex. 2Com- 2.0 10.0 1.7  6 1 ⊚ ∘ ⊚ Δ ∘ ⊚ Δ ∘ ⊚ Δ 0.32 0.31 0.31 140 Δ +3.6para Ex. 3 Com- 2.0 10.0 1.7 14 1 ⊚ x x Δ ∘ ∘ ∘ x x Δ 0.23 0.23 0.22  60∘ +1.8 para Ex. 4 Com- 2.0 10.0 2.9 14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ 0.21 0.210.20 120 Δ +1.8 para Ex. 5 Com- 2.0 10.0 1.0 18 4 Δ ⊚ ⊚ ⊚ Δ Δ ∘ ⊚ ⊚ ⊚0.23 0.23 0.22  50 ∘ −0.7 para Ex. 6 Com- 2.0 10.0 1.7 14 1 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚⊚ ⊚ ⊚ ⊚ 0.21 0.21 0.20 110 Δ +1.8 para Ex. 7 Com- 2.0 10.0 1.7 17 4 x ⊚⊚ ⊚ ∘ ∘ ⊚ ⊚ ⊚ ⊚ 0.19 0.19 0.18  40 ∘ −0.1 para Ex. 8 Com- 2.0 10.0 1.714 4 Δ Δ Δ x Δ Δ x ∘ ∘ x 0.22 0.22 0.28  60 ∘ +1.8 para Ex. 9 Com- 2.010.0 1.7 14 1 ⊚ Δ Δ Δ Δ Δ Δ Δ Δ Δ 0.22 0.22 0.24  60 ∘ +1.8 para Ex. 10Com- 2.0 10.0 1.7 14 1 ⊚ Δ Δ Δ Δ Δ Δ Δ Δ Δ 0.24 0.26 0.23  60 ∘ +1.8para Ex. 11 Com- 2.0 10.0 1.7 15 2 ∘ Δ x x Δ Δ x Δ x x 0.27 0.27 0.28 60 ∘ +1.5 para Ex. 12 Com- 2.0 10.0 2.0 16 1 ⊚ ⊚ ⊚ ∘ ⊚ ⊚ ∘ ⊚ ⊚ ∘ 0.210.21 0.22 110 Δ +1.3 para Ex. 13 Com- 2.0 10.0 1.7 26 5 x ∘ ∘ ∘ ∘ Δ ∘ ∘∘ ∘ 0.20 0.20 0.21  80 ∘ −2.0 para Ex. 14 Com- 0.3 13.4 0.3  7 4 Δ Δ Δ ΔΔ Δ Δ Δ Δ Δ 0.30 0.30 0.32 190 x +3.7 para Ex. 15 Com- 4.0  6.0 3.3 17 3Δ ⊚ ⊚ ⊚ ∘ ∘ ⊚ ⊚ ⊚ ⊚ 0.20 0.20 0.19  50 ∘ 0 para Ex. 16 Com- 14.0  —11.7  17 3 Δ ⊚ ⊚ ⊚ ∘ ∘ ⊚ ⊚ ⊚ ⊚ 0.20 0.20 0.19  50 ∘ 0 para Ex. 17

What is claimed is:
 1. A biaxially oriented laminated polyester film formagnetic recording medium, the biaxially oriented laminated polyesterfilm comprising a polyester layer A containing fine particles on atleast one surface of a polyester layer B, wherein the fine particles inthe polyester layer A are composed of 0.001 to 0.03% by weight ofcrosslinked polymer particles (a) having an average particle diameter(da) of 0.9 to 1.6 μm, 0.1 to 0.8% by weight of inert particles (b)having an average particle diameter (db) of 0.4 to 0.8 μm and 0.05 to1.0% by weight of inert inorganic particles (c) having an averageparticle diameter (dc) of 0.01 to 0.3 μm and a Mohs hardness of 7 orabove, wherein the ratio of the thickness TA (μm) of the polyester layerA to the average particle diameter da (μm) of the crosslinked polymerparticle (TA/da) is in the range of from 0.4 to 2.0, the center lineaverage roughness RaA on the surface of the polyester layer A is from 8to 18 nm, and the windability index of the polyester film is 100 orlower at a take-up speed of 200 m/min.
 2. The biaxially orientedlaminated polyester film for magnetic recording medium described inclaim 1 wherein the crosslinked polymer particle (a) has an apparentYoung's modulus of from 10 to 100 kgf/mm².
 3. The biaxially orientedlaminated polyester film for magnetic recording medium described inclaim 1 wherein the average particle diameter (da) of the crosslinkedpolymer particles (a) is from 1.0 to 1.5 μm.
 4. The biaxially orientedlaminated polyester film for magnetic recording medium described inclaim 1 wherein the content of the crosslinked polymer particles (a) isfrom 0.003 to 0.025% by weight.
 5. The biaxially oriented laminatedpolyester film for magnetic recording medium described in claim 1 or 2wherein the crosslinked polymer particle (a) is at least one kind ofparticle selected from crosslinked silicone resin and crosslinkedpolystyrene.
 6. The biaxially oriented laminated polyester film formagnetic recording medium described in claim 1 wherein the averageparticle diameter (db) of the inert particle (b) is from 0.4 to 0.7 μm.7. The biaxially oriented laminated polyester film for magneticrecording medium described in claim 1 or 6 wherein the content of theinert particles (b) is from 0.15 to 0.7% by weight.
 8. The biaxiallyoriented laminated polyester film for magnetic recording mediumdescribed in claim 1 or 6 wherein the inert particle (b) is calciumcarbonate.
 9. The biaxially oriented laminated polyester film formagnetic recording medium described in claim 1 wherein the averageparticle diameter (dc) of the inert inorganic particles (c) is from 0.03to 0.25 μm.
 10. The biaxially oriented laminated polyester film formagnetic recording medium described in claim 1 or 9 wherein the contentof the inert inorganic particle (c) is from 0.1 to 0.7% by weight. 11.The biaxially oriented laminated polyester film for magnetic recordingmedium described in claim claim 1 or 9 wherein the inert inorganicparticles (c) is at least one kind of agglomerated particle selectedfrom aluminum oxide and spinel-type oxide, and having an averageagglomeration degree of from 2 to
 20. 12. The biaxially orientedlaminated polyester film for magnetic recording medium described inclaim 1 wherein the ratio of the thickness TA (μm) of the polyesterlayer A to the average particle diameter (da) (μm) of the crosslinkedpolymer particle (a) (TA/da) is from 0.6 to 1.9.
 13. The biaxiallyoriented laminated polyester film for magnetic recording mediumdescribed in claim 1 wherein the laminated film has a Young's modulus of400 kg/mm² or above in longitudinal direction and 500 kg/mm² or above inlateral direction.
 14. The biaxially oriented laminated polyester filmfor magnetic recording medium described in claim 1 wherein the totalthickness of the laminated film is from 5 to 25 μm.
 15. The biaxiallyoriented laminated polyester film for magnetic recording mediumdescribed in claim 1 wherein the polyester is a polyethyleneterephthalate or a polyethylene-2,6-naphthalate.